Fine grained permissions on User Mapping

Paul Bonaud <paul@bonaud.fr> writes:

Imagine you have a destination database which you have no control over.
Let's call it “external-db”. This database has a unique pg user (no
specific pg permission attributes) with read-write access to the whole
database let's call it “external-user”.
...
Now over to our own database which we have control over. Imagine we want to
use a pg foreign data wrapper to access tables from the “external-db” from
a basic (non superuser) user, let's call it “basic-user”.
...
*However*, we would like to avoid our “basic-user” to have full control
over the external-db. We would like this basic user to only be able to
*read* the external database.
With this current setup the user can very simply list the user mappings
with details (\deu+ in psql) to collect the username/password combination
and thus directly connect to the initial “external-db” with full access.

So you're doing it wrong at at least two levels here:

1. The remote user you're mapping to ought to have just the privileges
you want the local user to have w.r.t. that database. User IDs are
cheap in Postgres; don't be so miserly as not to create a separate one
for each privilege level you need. If you did that, you wouldn't really
care whether the user could also connect directly to the remote DB.

2. You don't want to grant USAGE on the foreign server to the local
user, either. It's possibly an error in the design of SQL/MED that
foreign server USAGE grants both the ability to create/delete foreign
tables and the ability to create/delete/inspect user mappings. But
that's how the committee did it, so we're stuck.

If it's really too painful to not let the local user create/delete his
own foreign tables, then what you could do is make sure the remote user
ID's password is useless for any purpose except connecting from the
source database. One way to do that is to adjust the remote DB's
pg_hba.conf to disallow the remote user ID from connecting from
anyplace except the local database server.

regards, tom lane

Hi Tom,

Thank you very much for your answer.

I was worried to get this kind of solution, i.e. “don't be so miserly as
not to create a separate one for each privilege level you need.”, however
in the case of a remote database **you have no control over** it sounds
pretty impossible to do.

If I understand correctly, my initial question doesn't have a solution
within postgres, does this sound right?

Thanks again !
Paul

On Tue, 2 Jun 2020 at 16:08, Tom Lane <tgl@sss.pgh.pa.us> wrote:

On 6/3/20 4:11 AM, Paul Bonaud wrote:

Hi Tom,

Thank you very much for your answer.

I was worried to get this kind of solution, i.e. “don't be so miserly as
not to create a separate one for each privilege level you need.”,
however in the case of a remote database **you have no control over**it
sounds pretty impossible to do.

If I understand correctly, my initial question doesn't have a solution
within postgres, does this sound right?

As it stands now I can't think of one. You might reach out to the
postgres_fdw folks and see if they could get it to use a service file:

https://www.postgresql.org/docs/12/libpq-pgservice.html

Then the user mapping could use information the end user can't see
unless they had permissions on the file system.

Thanks again !
Paul
**

--
Adrian Klaver
adrian.klaver@aklaver.com

Hi again Tom,

I re-read your point 2. “You don't want to grant USAGE on the foreign
server to the localuser, either.” to find out this was exactly the solution
I was looking for. That is: it's fine to not let the basic user create the
foreign tables.

Wow, it was as easy as moving the foreign table creations up to a higher
“admin” level and giving only classical “select” grants to my local “basic”
user. That's it, it works! When the basic user tries to list the existing
user mappings of the database the “FDW options” column is now empty thus
not revealing the remote server's username and password.

Thank you very much !

On Wed, 3 Jun 2020 at 22:22, Adrian Klaver <adrian.klaver@aklaver.com>
wrote: